Danish patent specification No. 129,556 discloses a process for membrane filtration of milk or milk products for the preparation of an aqueous concentrate with a high content of protein substances, said process being carried out without additions of enzymes or chemicals such as acids or bases.
Journal of the Society of Dairy Technology, vol. 35, No. 2, April 1982, page 43 and the following pages, discloses ultrafiltration of non-curdled preacidulated milk. As it appears from the article on page 46, it is not of particularly great importance to the filtration capacity whether the filtration is carried out on pre-acidulated or normal milk.